It is known in connection with machine guns to surround the barrel with a covering filled with water so as to keep the temperature of the barrel as low as possible during the firing, and thereby to obtain a maximum period of continuous firing. It is furthermore known from the "Madsen" gun developed at the turn of the century by the Danish inventor Madsen to provide the barrel with ribs allowing an air cooling thereof, and to surround the barrel with a protective covering comprising large open holes. The holes both ensure a sufficient supply of air to the barrel from all sides and prevent the men from touching the hot barrel after the firing. Such coverings have previously only been used for small calibre weapons.
It has turned out in connection with large guns, such as for instance guns of a calibre of 40-120 mm or more where the gun is mounted in the gun turret of a tank, or in connection with anti-tank guns and ship guns for direct firing, that the effects on the gun barrel of sun, wind and rain from one side cause an undesired deflection reducing the accuracy of the gun, especially when firing the decisive first shot. In connection with a gun barrel of a length of about 3 m the deflection can be as much as about 3 mm, which deviates from the line of sight to such an extent that the possibility of a hit in the first decisive shot revealing the position of the gun is considerably reduced.
The German Offenlegungsschrift No. 36,39,866 discloses a gun barrel provided on the outer wall with a cooling duct. The cooling duct comprises a helical groove shaped in and on, respectively, the outer surface of the barrel, and furthermore a thin pipe pushed on said gun barrel and closing the grooves to form the ducts. A for instance bellow-shaped air distribution chamber is provided adjacent the chamber end, said distribution chamber blowing air from a blower through the ducts open at the muzzle end. A heat-insulating layer is provided between the gun barrel and the thin pipe. Such a structure is relatively expensive and complicated, and unless the blower is operating continuously, such a gun barrel is still subjected to deflection by the temperature effects from one side. In addition both the blower and the distribution chamber are subjected to shock effects which involves a high risk of malfunctioning.
German Offenlegungsschrift No. 1,918,422 discloses a gun barrel surrounded by a metal casing, which is situated at a short distance from the outer surface of the gun barrel. The still air between the barrel and the casing serves as heat insulation. Such as solution is, however, not very good because when the gun barrel is subjected to temperature effects from one side for a long time, said gun barrel will have a tendency to deflect.